MP1591 – 2A, 32V, 330KHz STEP-DOWN CONVERTER
APPLICATION INFORMATION
The inductance value can be calculated by the
equation:
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output voltage to FB.
The voltage divider divides the output voltage
down by the ratio:
(VIN − VOUT
)
L1 = VOUT
×
(VIN × f × ∆I)
Where VIN is the input voltage, f is the switching
frequency and ꢁI is the peak-to-peak inductor
ripple current.
R2
VFB = VOUT
×
(R1+ R2)
Table 1 lists a number of suitable inductors
from various manufacturers.
Where VFB is the feedback voltage and VOUT is
the output voltage.
Table 1—Inductor Selection Guide
Thus the output voltage is:
Package
Dimensions
(R1+ R2)
VOUT = 1.230 ×
(mm)
Vendor/
Model
Core
Type
Core
Material
R2
W
L
H
A typical value for R2 can be as high as 100kꢀ,
but 10kꢀ is recommended. Using that value, R1
is determined by:
Sumida
CR75
Open
Open
Ferrite
Ferrite
7.0 7.8 5.5
7.3 8.0 5.2
5.5 5.7 5.5
5.5 5.7 5.5
6.7 6.7 3.0
CDH74
R1 ≅ 8.18 × (VOUT − 1.230)
CDRH5D28 Shielded Ferrite
CDRH5D28 Shielded Ferrite
CDRH6D28 Shielded Ferrite
For example, for a 3.3V output voltage, R2 is
10kꢀ, and R1 is 17kꢀ.
Inductor (L1)
CDRH104R Shielded Ferrite 10.1 10.0 3.0
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor results in less ripple current that results
in lower output ripple voltage. However, the
larger value inductor has a larger physical size,
higher series resistance, and/or lower
saturation current. Choose an inductor that
does not saturate under the worst-case load
conditions. A good rule to use for determining
the inductance is to allow the peak-to-peak
ripple current in the inductor to be
approximately 30% of the maximum load
current that the IC can provide. Also, make sure
that the peak inductor current (the load current
plus half the peak-to-peak inductor ripple
current) is below the 2.3A minimum current
limit.
Toko
D53LC
Type A
Shielded Ferrite
Shielded Ferrite
5.0 5.0 3.0
7.6 7.6 5.1
D75C
D104C
Shielded Ferrite 10.0 10.0 4.3
D10FL
Open
Ferrite
9.7 1.5 4.0
Coilcraft
DO3308
DO3316
Open
Open
Ferrite
Ferrite
9.4 13.0 3.0
9.4 13.0 5.1
Input Capacitor (C1)
The input current to the step-down converter is
discontinuous, and so a capacitor is required to
supply the AC current to the step-down
converter while maintaining the DC input
voltage. A low ESR capacitor is required to
keep the noise at the IC to a minimum. Ceramic
capacitors are preferred, but tantalum or low
ESR electrolytic capacitors may also suffice.
MP1591 Rev. 2.3
9/27/2006
www.MonolithicPower.com
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